This application claims priority to U.S. Provisional Application No. 61/622,279, which is incorporated herein, in its entirety, by reference.
Human Respiratory Syncytial Virus (RSV) is the most common cause of upper and lower respiratory tract infections among infants and young children (premature infants especially prone to disease). World-wide, RSV is the leading cause of serious lower respiratory infections in infants (especially among those born prematurely and having chronic lung diseases or congenital heart diseases) and young children worldwide, and is responsible for a variety of illnesses, including 20-25% of pneumonia cases and 45-50% of bronchiolitis cases among hospitalized children. In the U.S., RSV invention results in approximately 120,000 hospitalization and 500+ deaths each year. In the population of <1 year old, RSV is the leading cause of infant viral death and mortality among this population is 10 times greater than mortality due to influenza infection. Global annual morbidity and mortality estimated to be 64 million and 160,000 deaths respectively. RSV-related medical costs estimated to be >$650 million/year.
While essentially all children experience an infection by two years of age, the peak age for serious RSV infection is at 2-6 months of age. The majority of infections resolve uneventfully. In some children, however, infection may predispose children for asthma and airway hyper-responsiveness later in life. Natural RSV infection does not confer lifetime immunity, and therefore, individuals may be repeatedly infected.
RSV infection raises significant issue among the elderly and other vulnerable populations. Among the elderly, RSV is the second leading cause of viral death. Transplant recipients and other immunocompromised populations as well as individuals suffering from cystic fibrosis are vulnerable to serious health consequences due to infection.
Respiratory Syncytial Virus (RSV) is an enveloped, negative-sense, single-stranded RNA virus of the family Paramyxoviridae, a family which includes common respiratory viruses such as influenza and those causing mumps and measles. In all, RSV has ten genes encoding eleven different proteins. The eleven RSV proteins include: proteins 1) protein “NS1” and 2) protein “NS2”, which inhibit type I interferon activity; 3) protein “N”, the nucleocapsid protein which associates with RNA forming nucleocapsid; 4) protein “P”, which is a cofactor for protein L; 5) protein “M”, the matrix protein which required for viral assembly; 6) protein “SH”, which is expressed on the viral surface forms the viral coat with protein G and protein F; 7) protein “G”, which is highly glycosylated, expressed on the surface, involved in viral attachment and binds glycosaminoglycans (GAGs); 8) protein “F”, which is expressed on the surface, viral-cell membrane fusion and mediating fusion to allow entry of virus into the cell cytoplasm; 9) protein “M2-1”, which is a matrix protein and elongation factor; 10) protein “M2-2”, which is a matrix protein and transcription factor; and, 11) protein “L”, which is RNA polymerase. The M2 gene encodes both protein M2-1 and protein M2-2 in overlapping open reading frames. The primary CD8 T cell epitope is encoded by the M2 gene. There are two major subtypes of human RSV—A and B. The major difference between the two subtypes resides within the G protein.
RSV infection often results both immune mediated pathoglogy and virus mediated pathology. Primary RSV infection often results in acute bronchiolitis that leads to inflammation-induced airway obstruction. RSV F binding has been shown to induce apoptosis resulting in the sloughing of ciliated epithelial cells, compromised pulmonary clearance, and consequent secondary infections.
Unfortunately, despite the immense effort, there are still no effective vaccines available for RSV. In 1966-1967, first RSV vaccine candidate, a formalin-inactivated alum-precipitated RSV preparation (FI-RSV vaccine) resulted in enhanced disease in vaccinated children upon subsequent natural infection. Histological analysis of lungs of children who died from enhanced disease caused by infection after vaccination revealed extensive mononuclear cell infiltration including pulmonary eosinophilia. The FI-RSV vaccine generated only binding antibodies without neutralizing activity because of denatured F protein and did not induce CTL activity.
Subsequent experiments have suggested that this enhanced pulmonary disease is associated with an exaggerated Th2-type cytokine response by CD4 T cells, a poor cytolytic response by CD8 T cells, and a weak neutralizing antibody response. RSV infection of FI-RSV vaccinated BALB/c and C57BL/6 mice resulted in enhanced disease observed in FI-RSV vaccinated children. Characteristic of a Th2-mediated immune response suggested immunized children were primed for Th2 immune response by vaccine. Increased levels of Th2-associated cytokines IL-5, IL-4, and IL-13 and chemokineeotaxin with a decrease in Th1-associated cytokine IL-12 were exhibited. Depletion of IL-4, IL-10, or IL-13 resulted in significant decrease in enhanced disease after RSV challenge.
Live attenuated and inactivated whole virus vaccines have also failed to protect. The candidate vaccines were either insufficiently attenuated or demonstrated the potential for enhanced disease. In 1982, a live attenuated RSV vaccine was found to be safe, but not effective for prevention of RSV illness.
In 1983, a Native American infant, “Baby Moose”, who was thought to have B streptococcal disease but who actually was infected with RSV, serendipitously improved when he received IGIV. This result prompted study of IGIV for RSV disease. In the mid 1980s through 1990 studies of standard IGIV for treatment and prevention of RSV illness validated the role of antibodies in prevention of RSV disease.
In the early 1990s, RSV vaccine studies were re-initiated using various subunit varieties. These trials failed to show significant protection from disease.
From mid 1990s to early 2002, clinical trials of palivizumab (monoclonal antibody specific for RSV-F) for prevention of serious respiratory tract disease caused by RSV produced positive results. In September 2003, palivizumab was approved for prevention of RSV-associated disease in high-risk children. Prophylactic treatment with palivizumab is effective in reducing the severity of disease, but is only recommended for high-risk patients due to the high cost involved in the treatment.
Although past studies have failed to yield in effective RSV vaccines, they have convincingly demonstrated the importance of immune responses in providing a thorough protection against RSV infection. Studies have found evidence supporting the importance of humoral responses and antibodies in protection against RSV-mediated disease. The presence of IgG antibodies in the lung directly correlates with reduced viral load and children with less severe RSV disease often have significantly higher anti-RSV antibody titers before infection Antibody to fusion protein is an important correlate of immunity. Infants who did not become infected with RSV had higher mean titers of IgG than infected infants and were born to mothers who had significantly higher maternal RSV-specific IgG antibody levels than the mothers of infants who became infected. The importance of antibody in mediating protection helps explain why premature infants are at such high risk for serious illness after RSV infection. Maternal IgG is not efficiently transferred to the fetus until the third trimester of pregnancy.
Similarly, the importance of cellular immune responses in providing a thorough protection against RSV infection has also been demonstrated. Children with T cell deficiency have difficulty clearing the virus and are more susceptible to subsequent RSV infection. In animal studies, depletion of CD8 T cells alone in mice does not result in chronic infection, but does result in delayed viral clearance. Clearance requires IFN-γ, FasL, TNF-α. Deficiency in any of these results in delayed viral clearance. Further, C57BL/6 mice immunized with VV-G do not develop RSV-associated enhanced disease, but depletion of CD8 T cells before immunization results in disease.
There remains a need for an RSV vaccine, including an immunogenic antigen, that induces long-term protection against RSV. There is also a need for a cost-effective delivery system to enable mass prophylactic vaccination against RSV. There is also a need for additional therapeutic agents to treat individuals infected with RSV.